These days it is expected to further reduce the thickness of a semiconductor chip in connection with diffusion of IC cards, mobile communication equipment and so on, or the miniaturization and reduction of thickness of electronic instruments. The thickness of a semiconductor chip has been around 300 μm heretofore, but it has been required to reduce thickness to 150 μm or less in some cases.
A semiconductor chip is generally manufactured through steps of: adhering an adhesive film for protecting the surface of a semiconductor wafer on the surface of a semiconductor wafer where circuits are formed; reducing the thickness of the semiconductor wafer by processing the non-circuit-formed surface thereof; peeling the adhesive film for protecting the surface of a semiconductor wafer; and finally dicing the semiconductor wafer for finishing. Especially as for the non-circuit-formed surface processing in manufacturing a semiconductor chip with the thickness less than 150 μm, the first step is reducing the thickness to about 200 to 150 μm by the conventional grinding process, then sometimes further reducing the thickness by polishing, chemical etching and so on. Lately has been developed a technique to reduce the thickness of a semiconductor wafer to as thin as only 25 μm by a back grinding process and without polishing or chemical etching.
However, such a thin-layered semiconductor wafer may have a significant degree of warpage deformation because of its deteriorating rigidity, which becomes a manufacturing problem. In a process of reducing the thickness of a semiconductor chip in general, a semiconductor wafer is taken out of a wafer cassette by a robot and fixed to a jig which is called a “chuck table” in the processing machine to conduct the non-circuit-formed surface processing. The wafer is, after the non-circuit-formed surface processing, carried back into the wafer cassette or carried away to the next manufacturing process. If the warpage deformation of the wafer is significant during these processes, some of the processing may come to a halt because the wafer is damaged or because the robot can not convey the wafer, and so on. Also in the peeling process of the adhesive film for protecting the surface of a semiconductor wafer, serious problem such as the damage of wafers may occur because of flattening the wafer by force in fixing it to the “chuck table” in the peeling machine.
A thinning-processed semiconductor wafer may cause a warpage deformation. It is said to occur by the residual stresses of adhesive film for protecting the surface of a semiconductor wafer which is adhered on the wafer surface and of the protection film for circuits which is mounted on the wafer surface. The residual stress of adhesive film for protecting the surface of a semiconductor wafer occurs by tensile force which is applied to said adhesive film when it is adhered to the surface of semiconductor wafers. Generally speaking, adhesive film for protecting the surface of a semiconductor wafer using a stretchy, flexible substrate film tends to cause a big residual stress, resulting in an easy occurrence of warpage in semiconductor wafers.
On the other hand, the residual stress applied to circuit protection films is remarkable in the case of polyimide group protection films. Especially, if the polyimide group protection film is thick in the process of reducing the thickness of a semiconductor wafer, the warpage of the wafer increases by the residual stress of said polyimide group protection film, which may result in serious problems such as the halt of manufacturing processes caused by the damage of wafers or by incapability of conveyance by robots and so on.
As a method to decrease such warpage of thinning-processed wafers, various approaches have been examined by decreasing the residual stress applied when adhering the adhesive film for protecting the surface of a semiconductor wafer to the surface of semiconductor wafers. For example, in Japanese Laid-open Patent Publication No.2000-150432 is disclosed an adhesive film for protecting the surface of a semiconductor wafer using substrate film with high percentage of stress relaxation, and in No.2000-212524 is disclosed an adhesive film for protecting the surface of a semiconductor wafer using substrate film with high elastic modulus, respectively.
Though the adhesive film for protecting the surface of a semiconductor wafer using the substrate film with high percentage of stress relaxation disclosed in Japanese Laid-open patent Publication No.2000-150432 is effective to decrease the residual stress applied in adhering it to the surface of semiconductor wafers, it can not avoid any warpage of the semiconductor wafers which is caused by residual stress of circuit protection films. The adhesive film for protecting the surface of a semiconductor wafer using substrate film with high linear elastic modulus disclosed in Japanese Laid-open Patent Publication No.2000-212524, on the other hand, is effective to straighten the warpage of semiconductor wafers, but it is difficult to peel from the surface thereof.